1. Field of the Invention
The present invention relates to an extreme ultraviolet light source apparatus generating an extreme ultraviolet light by irradiating a liquid metal target with a main pulse laser light after irradiating the target with a prepulse laser light, and a method of generating an extreme ultraviolet light.
2. Description of the Related Art
In recent years, along with a progress in miniaturization of semiconductor device, miniaturization of transcription pattern used in photolithography in a semiconductor process has developed rapidly. In the next generation, microfabrication to the extent of 70 nm to 45 nm, or even to the extent of 45 nm and beyond will be required. Therefore, in order to comply with the demand of microfabrication to the extent of 32 nm and beyond, development of such on exposure apparatus combining an extreme ultraviolet (EUV) light source for a wavelength of about 13 nm and a reflection-type reduction projection optical system is expected.
As the EUV light source, there are three possible types, which are a laser produced plasma (LPP) light source using plasma generated by irradiating a target with a laser beam, a discharge produced plasma (DPP) light source using plasma generated by electrical discharge, and a synchrotron radiation (SR) light source using orbital radiant light. Among these light sources, the LPP light source has such advantages that luminance can be extremely high as close to the black-body radiation because plasma density can be made higher, luminescence only with a desired wavelength band is possible by selecting a target material, there is no construction such as electrode around a light source because the light source is a point light source with nearly isotropic angular distributions, and extremely wide collecting solid angle can be acquired, and so on. Accordingly, the LPP light source is expected as a light source for EUV lithography which requires more than several dozen to several hundred watt power.
In the EUV light source apparatus with the LPP system, firstly, a target material supplied inside a vacuum chamber is irradiated with a laser light to be ionized and thus generate plasma. Then, a cocktail light with various wavelength components including an EUV light is emitted from the generated plasma. Consequently, a desired wavelength component, which is a component with a 13.5 nm wavelength, for instance, is reflected and collected using an EUV collector mirror which selectively reflects the EUV light with the desired wavelength, and inputted to an exposure apparatus. On a reflective surface of the EUV collector mirror, a multilayer coating with a structure in that thin coating of molybdenum (Mo) and thin coating of silicon (Si) are alternately stacked, for instance, is formed. The multilayer coating has a high reflectance ratio (of about 60% to 70%) for the EUV light with a 13.5 nm wavelength.
Japanese patent application Laid-Open No. H11-250842 discloses a laser plasma light source which is a light source with a high conversion efficiency due to previously forming a trench on a solid target, gasifying a surface portion of inside the trench by irradiating the trench with an ablation laser, and thermal ionizing the gasified material by irradiating this material with a heating laser light. Here, conversion efficiency means a ratio of power of a generated EUV light with a desired wavelength to power of a laser light that entered a target. This light source is suitable for temporary observation of the extreme ultraviolet light because the conversion efficiency can be made higher. However, because of using a bulk material, it is difficult to continuously form the same trenches over a long time and continuously supply the solid targets to the trenches. Therefore, it is difficult to use the light source as a light source for the exposure apparatus that is required to drive stably over a long time.
Published Japanese Translation No. 2005-525687 of the PCT International Publication discloses a certain apparatus. In this apparatus, a second target is generated by irradiating a first target at a gaseous state which could be noble gas such as Xe, the noble gas being gas under ordinary temperature that is discharged from a nozzle by a pressure, with a first energy pulse. Then, the second target isotropically-diffused with time is irradiated with a second energy pulse in order to generate a plasma, and a radial ray is outputted from the generated plasma. However, in the case where the Xe target is used, because luminance efficiency of the desired EUV light with a 13.5 nm wavelength is low, the conversion efficiency becomes low (under 1%). Therefore, it is difficult to use such apparatus using the Xe target as a light source for the exposure apparatus.
On the other hand, US patent application Laid-Open No. 2008/0149862 discloses a laser light source using a liquid droplet C300 of Sn which is able to efficiently emit a 13.5 nm EUV light as a target. In this laser light source, firstly, as shown in FIG. 1(a), a target 300 being a liquid droplet is broken and expanded by a prepulse P300. After that, the expanded target 301 is irradiated with a main pulse P301, and an EUV light is generated. In this light source, it is possible to improve the conversion efficiency of the EUV light by using the Sn as a liquid droplet but not solid, and irradiating the shattered and expanded target with the main pulse. Thereby, the light source is able to drive stably over a long time as the laser light source for the exposure apparatus. Moreover, the conversion efficiency is improved because of using a liquid metal but not gaseous as a target.
However, when the droplet being a liquid metal is irradiated with the prepulse, the droplet will be flicked off from a droplet moving axis to be broken. Therefore, a plasma, gasified droplets and tiny droplets, or the like, will drift along an optical axis of the prepulse laser while expanding, and a lot of debris can be generated. Furthermore, also due to irradiating these drifted and expanded targets with the main pulse even more debris can be generated and fly off, if a diameter and a yield of the tiny droplets is large. The flying debris contaminate or damage neighboring optical elements such as a reflecting surface of an EUV collector mirror, for instance, and thus can decrease a reflecting ratio with respect to a 13.5 nm EUV light in a short time. Therefore, there is a problem that a light source that can be reliable for a long time can not be provided.
In addition, because the flying clustered tiny droplets are neutral particles, flight thereof can not be controlled by generating an electromagnetical field, or the like. Moreover, the difficulties of controlling a position and distribution of such flight of targets makes it difficult to adjust a spot diameter of the main pulse to the targets. Therefore, as described above, most tiny droplets are not irradiated with the main pulse but fly off around directly as debris.